Asteroids that come within the orbit of Mars and pass close to Earth are collectively known as near-Earth objects (NEOs). The Lincoln Near Earth Asteroid Research (LINEAR) project was set up to detect near-Earth asteroids that may threaten our planet. An object two kilometers (one mile) across or larger likely strikes Earth about once in a million years. The large asteroid that struck Mexico's Yucatan peninsula 65 million years ago may have been responsible for the extinction of the dinosaurs as well as many other classes of species.

A few asteroids are so big that many astronomers have taken to calling them minor planets.